A turbofan gas turbine engine operates according to well known principles wherein an incoming air stream flows through the engine along an annularly configured, axially extending flow path. A portion of the incoming air stream is compressed in a compressor section of the engine and then mixed with fuel and burned in a combustor section to produce a high energy, high temperature exhaust gas stream. The gas stream exits the combustor and subsequently passes through a turbine section that extracts energy from the exhaust gas stream to power the compressor and produce bypass thrust by rotating a fan that acts generally on the remaining portion of the incoming air stream.
Uncontrolled leakages of gases within the engine contributes to a reduced engine efficiency. Seals are used to control this energy loss by interposing them in a leakage path to reduce the volume or mass of the gas--atmospheric air, exhaust, or otherwise--passing from one part of the engine to the other. In the past engine seals have principally taken the form of labyrinth seals. The use of brush seals as a substitute for labyrinth seals is presently being investigated.
A typical brush seal includes a plurality of seal stages with each stage including a plurality of bristles disposed between a pair of annularly configured plates. Usually the bristles are disposed at about a forty five degree angle to a radius drawn from the engine center line. A brush seal is usually attached along its outer circumferential edge to a stationary portion of the engine with the inward, free ends of the bristles disposed in a sealing engagement with a sealing surface on a rotating engine part. Because the bristles are somewhat flexible, they are able to bend during an engine transient and still retain their sealing ability after the transient has passed. Examples of such transients include differential thermal growth between the engine parts, rotor/stator relative movement, and vibration of some sort. Thus, a rotating engine shaft, for example, may enter a vibration mode where the shaft is vibrating about its longitudinal axis.
The sealing efficiency of a brush seal over time is affected by the wear on the bristle ends contacting the sealing surface on the opposing engine part, as well as the overall contact of the bristle ends with the sealing surface. Worn bristles ends will dictate replacement of the seal or particular seal stage earlier than otherwise would be necessary, thereby increasing engine operating costs. Because the bristles are not directed along true radii to the engine center line, but rather are angled at about forty five degrees relative thereto, excessive, irregular wear of the bristles may result from an adversary gas flow field, that is, a gas flow field that includes substantial velocity vectors disposed at angles other than perpendicularly to the seal. This adversary flow field can reduce the compactness of the bristle pack, which permits the individual bristles of the bristle pack to move randomly with a higher degree of freedom than the bristles of a seal not encountering such an adversary flow. When the bristles are tightly packed, they wear better over time and seal more efficiently. This enlarged freedom of bristle movement from the adversary flow field results in the bristles being displaced and rubbing, which in turn creates bristle wear.
Stated otherwise, any gas that encounters the seal that has a swirl, recirculation, or turbulence associated therewith will move the bristle ends and will contribute to unwanted bristle wear, often called tufting when originating from these causes. In common parlance, swirl is a rotational movement of the fluid molecules; recirculation is a radial movement of the fluid molecules; and turbulence is random, volatile movements of the fluid molecules. Thus, a radially outwardly directed recirculation, for example, can lift the bristles, which are usually attached at their radially outer but not their radially inner ends, thereby fluffing them and reducing their density. Additionally, an upstream jet flow, which forms part of the gas adversary flow, will vibrate the loosely packed or fluffed bristles and may open a small gap between the bristle ends and the sealing surface such that air can freely pass by an upstream seal stage and encounter a downstream seal stage with great velocity. This, in turn, will cause irregular wear on the downstream seal stage and may open a leakage gap at the adjacent downstream stage also, again allowing free passage of air or gas and greatly reducing engine efficiency.
It would be desirable to increase the lifetime and sealing efficiency of brush seals by reducing bristle wear caused by swirling, turbulent, recirculating air or gas flowing through the seal by reducing or eliminating the radially outwardly directed recirculating air or gas, the jet stream, or the turbulence associated with the air passing through the seal.